This happens to me more than I'd like to admit, but sometimes I don't notice the power getting low since it only shows up in my status bar, then the computer just quits. I'd like a big alert that warns me before this happens. Is there any way for it to alert me? Ubuntu has a nice popup that tells you it's getting low.

I had that same problem and I figured out a workaround. I installed from Linux Mint repositories an app called GKrellM. It monitors lots of stuff on your machine and there is battery. Setup with alarms, it works for me like a charm.

As I don't have Gnome Shell or Cinnamon installed, I cannot be of direct help as accessing the source could would be much practical since it's rather easy adding such a rudimentary extension with JavaScript.

You should open :

/usr/share/cinnamon/js/ui/status/power.js

and search for percentageText - subsequently, you ought to be able to add a pop-up message or alert.

The issue is related at KDE environment. I have the same problem with MintMAYA KDE_version. When I worked with Mint9_Gnome a very useful pop-up appeared when a certain level of low-power was reached (auto-configured out_of_the_box on fresh installs). Very very annoying being shutted down without advice and no help founded even googling a lot :(

re: hdgarrood 's answer, setting cron to run notify-send actually turned out to be painfully finicky. (I think crontab -e does persist across reboots by default, fortunately). I followed several guides, I'm not sure what exactly fixed it in the end, but here is my full setup for running the checker every 5 minutes: